Social media has been a-twittering and a de-facebooking for days over a so-called 13th constellation – Ophiuchus – after an astronomer noted that the astrological zodiac isn’t anywhere near the stars they’re named after. This isn’t entirely true, like most things.

And it’s not news to the hundreds of millions of Hindus who rely on a sidereal zodiac rather than one that somewhat arbitrarily aligns the beginning of the zodiac sign Aries with the spring Equinox. Indian astrology (jyotish) uses a sidereal zodiac which uses the same names for the signs as western astrology, but they are aligned with the constellations. In addition, Indian astrology also uses 27 asterisms.

”In tropical astrology, in vogue in the West for many hundreds of years, the start of the zodiac is taken to commence with the Spring Equinox. Owing to the astronomical phenomenon called precession, this starting point tracks backwards approximately 51″ of arc per year against the background of the stars.

“At one point, both sidereal and tropical zodiacs coincided – this was in the 3rd or 4th century AD. Since this date, the two zodiacs have inexorably separated from one another. The separation is not yet absolute as the signs share a common arc of approximately six degrees.

“When a tropical astrologer states that the Moon is in Aries 10 degrees, what she or he probably does not realise is that the Moon occupies a point in the constellation of Pisces of about 16 degrees. Siderealists and tropicalists talk and write as if they are referring to the same constellations, but a gap of 24 degrees separates the one from the other. A rough and ready method of converting between the zodiacs is by adding six degrees to tropical longitudes and substracting a sign.

“Astrological textbooks seldom draw attention to these disparities, yet when this matter is referred to, the defence seems to hold the position that as astrology is connected with symbolism, the zodiac has no real affinity with the position of the stars whatsoever. Such assertions bring justifiable ridicule on astrology from astronomers.”

There is a group of Western astrologers who also use the sidereal zodiac rather than the tropical zodiac – this is down to the work of an Irishman called Cyril Fagan. The problem with the sidereal zodiac is what star to use as the starting point – Hindus use a star in the asterism Revati which doesn’t seem to be there any more – some of them use Spica; the Western astrologers after Fagan fix the starting point of the zodiac as Aldebaran.

So if you’re a Hindu born in Delhi, and a Westerner asks you your star sign it’s more than likely that he or she is talking about a spot 24 degrees away from the Westerner. Added to that, Hindus often refer to their star sign as the position where the Moon is at birth, not the Sun. There was a time when astronomers were astrologers – Fagan believed, for example, that the Babylonians used a sidereal zodiac. Their computation of the celestial spheres wasn’t bettered until the late 19th century – astronomers were astonished to find ephemerides on clay tablets were more accurate than those moderns could compute.